1. The Field of the Invention
The present invention relates to mandrels used for fabricating hollow continuous filament wound vessels and tanks and methods of constructing such mandrels.
2. The Prior Art
Methods of constructing filament wound vessels, tanks, and containers are well known in the prior art. Typically, a rigid mandrel made of aluminum, fiberglass, or other high strength and relatively lightweight material, or the like is prepared and mounted on a filament winding machine so that the mandrel may be selectively rotated. The surface of the mandrel is coated with an appropriate mold release preparation and then wound with resin impregnated or coated filaments, such as glass, KEVLAR.RTM., graphite, nylon or boron fibers. Commonly, the winding progresses from end to end for an elongated shape or from side to side for a more spherical shape. When the desired thickness of the winding layers is achieved, the winding is stopped and the resin is cured.
In many cases, the resulting filament wound vessel is removed from the mandrel by cutting the vessel about its circumference, generally at a location near the center thereof. The two halves of the vessel are then removed from the mandrel and the halves joined and bonded together to form the desired vessel or tank. A short helical wind of a resin coated filament strand or roving may be made over the joint of the vessel in an attempt to further secure the two halves together.
Examples of prior art winding techniques and methods are disclosed in U.S. Pat. Nos. 3,386,872, 3,412,891, 3,697,352, 3,692,601, 3,533,869, 3,502,529 and 3,414,449.
Because of the joint in the completed vessel, an inherent weakness exists which may be the first to fail or fracture when the completed vessel is subjected to pressure or stress. Because of the weakness in the resulting vessel and the added labor costs associated with cutting the vessel and rejoining the two halves of the vessel, techniques have been developed which allow the fabrication of hollow vessels without the need to cut the vessel to remove it from the mandrel.
In some cases, for example, a hollow mandrel is designed to become an integral part of the completed fiber wound vessel. Disadvantageously, the intended use of the completed fiber wound vessel is often not compatible with retaining the mandrel as the interior of the vessel. Another technique involves using a mandrel which is destroyed once the vessel is formed. It will be appreciated that if a large number of a particular configuration of fiber wound vessel are to be fabricated, destroying the mandrel with each use is an exorbitantly expensive technique. Thus, reusable mandrels have been developed.
In some cases, segmented metal mandrels, which can be disassembled into small sections and then removed through an opening in the completed vessel, have been used. Disadvantageously, building a reusable metal mandrel is costly and time consuming. The difficulty of building a reusable segmented metal mandrel makes it too expensive for all but the most demanding applications of high volume vessel fabrication.
Another type of mandrel which has been used to produce seamless completed fiber wound vessels is a collapsible mandrel. Collapsible mandrels are hollow mandrels made of flexible, air tight materials such as a rubber which can be inflated while the vessel is being formed thereon and then deflated and removed through an opening in the completed vessel.
One collapsible mandrel which can be removed through an opening in a completed vessel is disclosed in U.S. Pat. No. 4,684,423 to Brooks. While the method of forming the mandrel and, the resulting mandrel structure, which are disclosed in the Brooks reference represented a great advance in the art, several disadvantages still remain. The Brooks reference requires that the resulting mandrel be cut in half to remove it from a rigid mandrel. Cutting and splicing the mandrel structure results in an inherently weaker and less desirable mandrel. Since the area at the resulting joint is weaker than the remaining structure, the joint often fails sooner than the other portions of the structure. Thus, the usable life of the mandrel is often unduly limited because of the presence of the joint.
Further drawbacks and disadvantages inherent in the structure and method disclosed in the Brooks reference include the additional labor which is required to cut and rejoin the mandrel. Moreover, since the outside surface of the mandrel determines the shape and uniformity of the interior surface of the completed fiber wound structure, a poorly formed seam in the collapsible mandrel can result in an inconsistent surface in the completed fiber wound hollow structure. Even though the use of collapsible mandrels to form seamless completed structures is known, for example as in the Brooks reference, the problems inherent in a mandrel which has been cut and spliced together has not been addressed in the art.
In view of the forgoing, it would be an advance in the art to provide a seamless, collapsible, and reusable mandrel structure and an accompanying method of forming the same.